1. Field of the Invention
The present invention relates to a method of enabling a tuneable laser to be characterized quickly.
The method can be applied for evaluating and selecting lasers with respect to emitted wavelength and to find good operation points systematically.
2. Description of the Related Art
Tuneable semiconductor lasers have a number of different sections through which current is injected, typically three or four such sections. The wavelength, power and mode purity of the lasers can be controlled by adjusting the current injected into the various sections. Mode purity implies that the laser is at an operation point, i.e., at a distance from a combination of the drive currents where so-called mode jumps take place, and where lasering is stable and side mode suppression is high.
In the case of telecommunications applications, it is necessary that the laser is able to retain its wavelength to a very high degree of accuracy and over very long periods of time, after having set the drive currents and the temperature. A typical accuracy in this respect is 0.1 nanometer and a typical time period is 20 years.
In order to be able to control the laser, it is necessary to map the behavior of the laser as a function of the various drive currents. This is necessary prior to using the laser after its manufacture.
Mapping of the behavior of a laser is normally effected by connecting the laser to different measuring instruments and then varying the drive currents systematically. Such instruments are normally power meters, optical spectrum analyzers for measuring wavelength and sidemode suppression, and line width measuring devices. This laser measuring process enables all of these parameters to be fully mapped as a function of all different drive currents.
One problem is that lasers exhibit hysteresis. As a result of the hysteresis, the laser will deliver different output signals in the form of power and wavelength in respect of a given drive current set-up, i.e., with respect to a given operation point, depending on the path through which the laser has passed with respect to the change in said drive currents, in order to arrive at the working point in question. Thus, this means that a given drive current set-up will not unequivocally give the expected wavelength or power.
In the case of a tuneable laser, the wavelength of the emitted light is determined mainly by the current or voltage across the tuning sections. The power emitted is controlled by current to the gain section of the laser or by the voltage across said section.
When characterizing a laser, all of the possible control combinations afforded by the tuning sections, or a subset of said sections, are investigated. During the characterizing process, the emitted light is studied with respect to wavelength and sidemode suppression and controlling the gain section with regard to power adjustment.
The enormous number of possible control combinations, typically tens of billions, of which fewer than a hundred will be selected, makes total mapping of the laser impossible in view of the large amount of data generated.
The present invention solves this problem and provides a method of quickly sorting away control combinations that do not result in correct wavelengths.
The present invention also relates to a method of evaluating a tuneable laser and determining suitable laser operation points for a laser that includes two or more tuneable sections, in which injected current can be varied and of which at least one is a reflector section and one is a phase section. The method includes leading part of the light emitted by the laser to an arrangement that includes a Fabry-Perot filter and a first and a second detector, said detectors being adapted to measure the power of the light and to deliver a corresponding detector signal. The detectors are arranged relative to the Fabry-Perot filter such that the data signals will contain information relating at least to the wavelength of the detected light. The currents are swept through the tuning sections so as to pass through different current combinations, and the ratio between the two detector signals during said sweeps is measured. The reflector current in the inner sweep variable is swept in one direction and then in the opposite direction back to its start value. The control combination for the tuning currents is stored when the ratio between the detector signals lies within a predetermined range that indicates that the light emitted lies within one of a number of wavelengths given by the Fabry-Perot filter, and when the ratio lies within said predetermined range for a given reflector current in both sweep directions of said current.